A computer-controlled, photon-counting microspectrophotometer will be used to measure in the retinas of freshwater turtles: (1) The pairings of oil droplet or other screening pigment type with visual pigment type in individual photoreceptors. (2) The morphology, the spectral absorptance of the visual pigment, and the spectral transmittance of the screening pigment, if any, in each individual receptor. Receptors will be classified by their combinations of these three characteristics. (3) The averaged spectral absorbances for each visual pigment type will be determined by fitting a template which consists of the sum of three Gaussian curves. Variations of the individual receptors from those averages will be determined. (4) The averaged spectral transmittance of the screening pigment will be determined for each receptor type, as well as the variations of individual receptors from those averages. (5) The product of the averaged spectral absorptance of the visual pigment and the spectral transmittance of the screening pigment will be determined for each receptor type to get its spectral sensitivity. (6) For each receptor type the variation in the individual receptor of its screening pigment from the average and of its visual pigment from the average will be examined for correlations. (7) The above will be done by species for each of the species, Emydoidea blandingii, Chelydra serpentina, Chrysemys picta, and Chrysemys scripta elegans. (8) The above will also be done by individual animal and by retinal region for the latter two species.